Simple movements to implement, such as shenage (spelt right?) are incredably easy to use from verbal/ first contact period of a fight, when person has grabbed onto shirt etc. using an aikido technique in a situation like this would count as self defence, so from personal experience, it is effective dependent upon circumstances.

Cooperation and going slow are an essential part of learning any martial art. Without those steps, no one could learn techniques properly.

However, in the end, techniques learned slowly in a cooperative environment have to be applied effectively against actively resisting opponents. And this is where aikido often fails. Most aikido training never gets beyond the cooperative phase. If you never apply what you are learning against actively resisting opponents, you are not really learning all that much.